334 NATURE STUDY AND AGRICULTURE 



4 ' theory of environment," for they believed that the effect 

 of environment would explain all the changes necessary in 

 passing from one species to another. They had observed 

 the seasonal changes in the plumage of birds and in the coats 

 of mammals, and inferred that plants and animals are 

 molded by changes in their environment, as clay can be 

 molded by the hand. This explanation was soon found 

 to be entirely inadequate, for the necessary changes are 

 too deep-seated to be brought about by such a superficial 

 agency. But "environment" has ever since played a very 

 real, even if a very subordinate, part in every evolutionary 

 theory. 



2. The next epoch was introduced in the first decades 

 of the nineteenth century by the announcement of the same 

 explanation by two men; Lamarck of France, and Tre- 

 viranus of Germany. It may be called the "theory of the 

 effect of use and disuse," although Lamarck called it the 

 "theory of appetency," which means the theory of desires. 

 It is based upon the fact that use develops an organ and dis- 

 use results in its dwindling and possible disappearance. 

 For the sake of illustration it is sometimes called "the 

 law of the blacksmith's arm," because it is well known that 

 such use of the arm develops the muscle, and that disuse 

 will cause a muscle to shrivel and lose its power of funo- 

 tioning. This explanation of evolution was thought to be 

 deep-seated enough, for it could apply to every organ of 

 the body. Lamarck, for example, imagined an animal, 

 adapted to certain conditions, transferred to new condi- 

 tions that would mean different demands. Certain organs 

 would be called upon persistently that were not so im- 

 portant under the old conditions, and would thus be 



